Gluten is a protein derived from cereals such as wheat and rye, and forms a peculiar gel having water retentivity and viscoelasticity, and has been used as raw materials for yakifu (baked bread-like wheat gluten), namafu (dumpling-like wheat gluten) and fugashi (wheat gluten bun) for a long time. Recently, gluten is widely used for increasing of the volume of breads, and used as a quality improver for noodles and fish paste products such as kamaboko (minced and steamed fish). Gluten is mainly obtained by adding water to wheat flour, kneading to make dough, and then washing away starch from the dough with water. The resultant gluten is called “fresh gluten” and is usually a gum-like lump containing 60 to 70% of moisture. Although fresh gluten is sometimes distributed in a frozen state after freezing, the product obtained by forming fresh gluten into a powder by drying is “active gluten”, which is widely distributed because of good keeping quality and low distribution cost.
Fresh gluten has poor water dispersibility and is a gum-like lump. Therefore, it is difficult to obtain active gluten by drying fresh gluten, and various pre-treatment steps are required. For example, there is a method (spray-drying method) in which fresh gluten is dispersed in water in advance using an inorganic acid such as hydrochloric acid and sulfuric acid, an organic acid such as acetic acid, an alkali such as ammonia and sodium hydroxide, and a reducing agent, followed by spray-drying. However, since the acid, alkali or reducing agent is used in the spray-drying method, flavor and color of gluten deteriorate, and physical properties such as gel strength become inferior.
Alternatively, there is a method (flash-drying method) in which fresh gluten is mixed with a dusting powder (active gluten is usually used) while finely cutting, thereby drying in an air current while adjusting moisture. In the case of the flash-drying method, since a drying treatment can be carried out within a short time, less influence is exerted on physical properties. However, there is an operation problem that it is difficult to mix a dusting powder with massive fresh gluten and excess heat is applied to the dusting powder, and therefore slight deterioration of quality cannot be avoided. There is also a method (freeze-drying method) in which fresh gluten is freeze-dried. Although freeze-drying method enables suppression of denaturation of active gluten and less change in color and physical properties, most detrimental defect is high drying cost. As described above, conventional methods for producing active gluten have problems in manufacturing technologies and quality of the resultant active gluten, and thus there has been required to develop a simple and easy method capable of providing active gluten having good quality at low cost.
In addition, it is difficult to add commercially available active gluten obtained through these drying steps during processing into various foods uniformly since it returns to a gum-like lump as a result of hydration. Usually, although active gluten is added to foods after mixing with a powder such as wheat flour, it becomes necessary to improve water dispersibility taking a complicated operation and cost into consideration. Patent Document 1 discloses a method for producing vital gluten flour in which water dispersibility is improved by adding lecithin into fresh gluten and then the mixture is uniformly emulsified and powderized by drying. However, the operation is complicated and it could not be said that the resultant active gluten has sufficient dispersibility.
Patent Document 2 discloses modified gluten flour (active gluten) containing a thickener. This patent document is designed to enhance and modify physical properties of gluten by the thickener and to impart texture such as hardness, glutinousness and smoothness to foods using the modified gluten flour, and the modified gluten flour is obtained by kneading fresh gluten containing 60 to 70% of moisture with a thickener, followed by drying and further milling, but is insoluble in water. This patent document does not teach a method in which gluten is dispersed in water by adding a specific polysaccharide to fresh gluten and mixing it.
In addition, there is a method in which a thickener is added to an acidic gluten dispersion solution to obtain an aggregate and the aggregate is dried and ground to obtain modified gluten flour (active gluten) (Patent Document 3). However, it is an essential requirement that an aggregate is obtained by bringing gluten into contact with a thickener in an acidic solution, and this patent document does not teach a method of dispersing gluten in water in the neutral region. As described above, with respect to a method of improving water dispersibility of fresh gluten and active gluten without deterioration of quality or complication of the step, a good method has never been developed.
A lot of methods of improving emulsifiability and foaming property by hydrolysis of gluten thereby improving functions have been proposed. Since wheat gluten contains a large amount of glutamine and glutamic acid in constituent amino acids, it is useful as raw materials for preparing glutamine peptide which is useful as an intestinal infusion solution material, or raw materials of an amino acid-based seasoning agent such as glutamic acid. For this reason, various studies are made for efficiently hydrolyzing gluten.
Wheat gluten is a macromolecule having a molecular weight of several hundreds, which is composed mainly of gliadin and glutenin, and is neither dissolved nor dispersed in water in the neutral pH region and becomes an aggregated lump having viscoelasticity. The method of hydrolyzing gluten is roughly classified into a degradation method using a mineral acid such as hydrochloric acid or sulfuric acid, and a degradation method using an enzyme.
According to the degradation method using an acid, gluten is hydrolyzed by dispersing in a mineral acid such as hydrochloric acid or sulfuric acid. However, since gluten is not sufficiently dispersed or an adhesive dispersion solution having high viscosity is prepared, and therefore the content of gluten is compelled to low. Because of insufficient stirring and nonuniform temperature, there have been problems that the undegraded gluten is remained, while a degraded product having extremely low molecular weight is produced, and also only a nonuniform product having wide distribution of molecular weight is obtained, and the undegraded gluten is remained resulting in low yield. There have been also problems that the resultant hydrolysate has unfavorable tastes such as pungent taste and bitterness which may be due to an acid.
There have been also problems in a degradation method using an enzyme. Since gluten is aggregated in the neutral pH region, when enzymatic hydrolysis is carried out in the neutral pH region, particularly when fresh gluten is used as a raw material, it is difficult to disperse fresh gluten and it requires a long time to sufficiently hydrolyze gluten, and thus it is not commercially practicable from an economical viewpoint. Alternatively, there is a method in which gluten is dispersed using an acid and the enzyme reaction is carried out in the acidic pH region. However, this method requires great care, in addition, the concentration cannot be increased because gluten cannot be sufficiently dispersed and remarkable thickening arises when using the acid. The resultant hydrolysate has pungent taste and bitterness due to the acid. When the hydrolysate is neutralized so as to remove the acid, saltiness is produced. To cope with these problems, various studies have been made so as to efficiently obtain a gluten hydrolysate having good quality.
For example, there is disclosed a method for obtaining a flavoring agent by dispersing gluten with a deamidating enzyme and then subjecting to a hydrolysis treatment (Patent Document 4). However, it is not easy to disperse gluten and a long reaction time is required. In addition, although a dispersion solution is not prepared, there is proposed a method for obtaining glutamine peptide efficiently by processing gluten into a porous dried granule in advance and then carrying out hydrolysis by using an enzyme (Patent Document 5). However, special processing such as freeze-drying is required so as to prepare porous gluten and the operation is complicated, and also it cannot be said that the method is not suited for practical use in view of production cost. In light of the above background, there has been required to develop a technology in which water dispersibility of gluten is improved by a simple and easy method to efficiently obtain a hydrolysate.    Patent Document 1: JP 62-14253 B    Patent Document 2: JP 2005-204649 A    Patent Document 3: JP 2007-46 A    Patent Document 4: JP 2000-515003 A    Patent Document 5: JP 2000-287698 A